Spontaneous wear and tear creates damaged DNAan abasic siteThese are common examples of spontaneous DNA damageuracil in DNADeamination of an intentionally 5meC, common in eukaryotes like us, gives rise to another normal base, T. Does T get repaired or stay mispaired with G?2

Different types of repair mechanismcorrect different types of DNA damageIn increasing order of the complexity of the problem:• direct repair of a speciﬁc base modiﬁcation• base excision repair when one base is missing or altered• (oligo)nucleotide excision repair when there is adistortion of B-form DNA with damage on one strand• mismatch repair when both bases are OK but their paired combination is not• error-prone repair of damage to one strand whenthe other is not present to provide a template• repair of a double-stranded DNA break3

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This is not wise as a general repair mechanism, because new or unusual types of damage will not be possible to repair. But, it can increase the efﬁciency of correcting common damage. For example, E. coli has a speciﬁc enzyme to remove the methyl group from O6-methylguanine, directly reversing the modiﬁcation. However, the enzyme commits ‘suicide’ by self-modiﬁcation and must then be degraded.

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